A variety of current measurement techniques are known, including resistive shunt, current transformer, Hall Effect based sensors and Magneto-resistive (MR) sensors. The shunt measurement technique works on the principle of Ohm's Law, while current transformer and Hall Effect based sensors utilize Ampere's Law. Each technology has its own tradeoffs. Shunts offer low cost, DC (Direct Current) and AC (Alternating Current) current sensing but insert a voltage drop and do not provide isolation. Current transformers are of low cost and additionally provide isolation, but only work for ac current. Hall Effect based sensors, both open and closed loop technology, provide isolation and DC to high frequency (200 kHz) AC operation, but have limitations in cost, size, linearity and temperature performance. The Magneto Resistive (MR) sensor uses a magneto resistor, which is a two-terminal device that changes its resistance with a change in magnetic field.
Presently, current sensor uses a magnetic field transducer (for example a Hall effect or Magneto Resistive (MR) transducer) in proximity to a current conductor. The magnetic field transducer generates an output signal having a magnitude proportional to the magnetic field induced by a current that flows through the current conductor. Some typical Hall Effect current sensors include a gapped toroid magnetic flux concentrator, with the Hall Effect element positioned in the toroid gap. The Hall Effect device and toroid are assembled into a housing, which is mountable on a printed circuit board. In use, a separate current conductor, such as a wire, is passed through the center of the toroid. Such devices tend to be undesirably large, in terms of height, weight and circuit board area.
Also known is that closed loop current sensors are based on the principles of Magneto-resistive technology and the null balance or zero magnetic flux method (feedback system). The magnetic flux in the sensor core is constantly controlled at zero. The amount of current required to balance zero flux is the measure of primary current flowing through the conductor. These sensors are used primarily for motor control applications and power electronics. The sensor generally utilizes an application-specific integrated circuit (ASIC) technology to exploit high performance for MR sensor. The sensor uses a magnetic core to concentrate the measured magnetic field around the primary conductor which protects the sensor from external magnetic interferences making it more accurate and reliable.
Besides providing accuracy and reliability core sensors have inherent demerits. Increased sensor weight and size, sluggish time response at higher frequencies and nonlinear permeability variation with frequency that can affect the output accuracy are some of the drawbacks of using current core sensors. Further the core saturation at higher currents, magnetic remanence (offset) and it's variation with current magnitude and temperature dependency of core magnetic properties can affect output accuracy. Furthermore, a sensor with core turns out to be more expansive due to material and assembly cost which is usually around 20% of product cost. As highly sensitivity magnetic sensors (MR, Hall etc.) are currently available, field concentration may not be required; however the usage of a core is still preferred by designers for the sensor's reliable operation under external magnetic interferences, and designers work around it's demerits to minimize them to suit an application requirement. An Ideal market demand and also a designer's dream is to have a current sensor which is core-less and also have protection from external magnetic interferences.
It is also known that a core-less current sensor comprising a U-type of parallel conductors with opposite direction currents is available in closed loop configuration. The primary current can be fed through a U-shaped conductor for creating a field gradient between the two sides of the conductor. Thin film MR resistors can be placed on a silicon chip and can be connected in a Wheatstone bridge. The chip can be mounted together with analogue interface electronics on a single in-line hybrid circuit. Compensation current can be fed back to the sensor chip through a compensation conductor located above MR resistor. The resulting field exactly compensates the field gradient so that the sensor always works around a single point. At the output of the sensors, the compensation current flows through a measurement resistor resulting in an output voltage.
In another known core-less current sensor technology, a precision, low-offset linear Hall sensor circuit with a copper conduction path located near the die is used. Current flowing through the copper conduction path generates a magnetic field which can be sensed by the integrated Hall (integrated Circuit) IC and converted into a proportional voltage. Device accuracy is optimized through the close proximity of the magnetic signal to the Hall transducer. In yet another core-less current sensor, a single-axis integrated magnetic field sensor based on the Hall Effect is used. The circuit can be fabricated using a conventional Complementary metal-oxide-semiconductor (CMOS) technology with an additional ferromagnetic layer. The ferromagnetic layer can be used as a magnetic flux concentrator providing a high magnetic gain.
The existing core-less current sensors are not immune from external interferences; therefore, a need exists for a core-less sensor that is immune from external magnetic interferences. Furthermore, the core-less sensor is preferred over a sensor utilizing core as it does not have the limitations on time response, frequency response, and have no magnetic remanence (offset), no saturation and non linearity issues. As will be shown with the foregoing invention, the present inventors have discovered that it is possible to achieve a low cost, light weight and small size core-less current sensor which is immune to external magnetic interferences.